06 September 2006
Cartilage and Bone
Dr. Robert Crissman, Ph.D.

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Cartilage

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• Supportive CT
• CT that makes up skeletal framework of body
• Consists of cartilage and bone
  • Provide support for other soft tissues

Cartilage

• Three type: hyaline, elastic, fibrocartilage
  • Names based on the predominant fiber present in the extracellular matrix
• Hyaline cartilage
  • Made of type II cartilage
  • Found where needed for weight bearing but some flexibility required
• Elastic cartilage
  • Made of elastic fibers
• Fibrocartilage
  • Made of type I collagen

Hyaline Cartilage

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• Most common type of cartilage
• Condrocytes
  • Diagnositic Characteristic: Large round cells well separated from other cells, avascular
  • Lots of extracellular matrix
  • Surrounded by perichondrium – dense irregular CT

Chondrocytes

• Reside in lacunae
  • Artifact: Cell shrinks away from matrix, leaving a “lake” during slide making; normally, cells are right up against matrix
• Maintains extracellular matrix
• Does not divide

Chondroblasts

• Two cells in one lacunae
  • Diagnostic Characteristic: Divides
• Produces additional new matrix
  • Two cells in one lacunae push each other apart as new matrix is being laid down

Cartilage Growth

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• Interstitial growth
  • “Growth from within”
  • Add new matrix within cartilage because extracellular matrix is flexible enough
• Appositional growth
  • Growth on the perimeter or sides of cartilage

Perichondrium

• Two layers
  • Fibrous Layer
    • Outer layer of dense irregular CT
    • Vascular
  • Chondrogenic layer
    • Differentiating into chondroblasts
    • Appositional growth
• Lots of proteoglycans so that H2O diffusion can occur
  • However, prevents wandering cells from going into cartilage

Ground Substance

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• Abundant, amorphous gel
• Rich in proteoglycans
• Territorial matrix (a.k.a. capsular matrix)
  • Immediately around cell
  • Darker staining due to chondroitin sulfate
  • Few collagen fibrils
• Interterritorial matrix
  • Between cells
  • Lighter staining matrix between cells because of lower condroitin sulfate content

Proteoglycans

• Aggrecan aggregates are what provides cartilage its characteristic structure and gel-like character
• More organized than CT proper
• Aggrecan aggregate is a hyaluronic acid with 100 aggrecan molecules attached
• Diffusion of nutrients and H2O
• Resists compression
• Slippery
  • Mother nature’s Teflon

Glycoproteins

• Adhesion molecules
• Chondronectin
  • Attaches cell membrane to ECM
• Chondrocalon
  • Binds Ca2+ to the ECM

Extracellular Fibers

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• Type II collagen doesn’t aggregate together to form fibrils or fibers
  • Cannot be seen at light microscopy level
  • Fine microfibers at electromicroscope level
    • Pericellular capsule visible adjacent to cell
      • Thought to protect cells form mechanic stress

Hyaline Cartilage Location

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• Costal cartilage
• Trachea, bronchi of lungs
• Larynx, nasal cartilage
• Articular cartilage of synovial joints
• Fetal skeleton
  • Allow flexibility during birth
  • Cartilage grows faster than bone
    • Has both appositional and interstional growth (bone is only appositional growth)

Functions of Hyaline Cartilage

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• Provide structural support
  • Elastic firmness that is resilient yet fairly rigid
• Withstand pressure and shear forces
  • Acts as shock absorber
  • Due to proteoglycan aggregates being full of water
• Slippery
  • 80% of weight is water
  • Due to proteoglycans
• Capable of rapid growth

Cartilage Formation

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• Can be formed directly from Mesenchyme
  • Areas of low O2 content causes formation of chondroblasts
    • In areas of high O2 content causes differentiation to bone
  • Called procartillage
  • Chondroblasts secretes matrix interstitially, pushing away from each other to form cartilage

Elastic Cartilage

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• Chondrocytes
  • Large round cells, widely scattered
• Principal fiber
  • Elastic fibers
  • Collagen Type II
  • Both fibers secreted by condrocyte
    • Special stain required to visualize elastic fibers
• Ground Substance
  • Same as hyaline cartilage
  • Lots of aggrecan aggregates
• Location
  • Auricle and epiglottis
• Function
  • Resilient support
• Diagnositic Characteristics: large, round chondrocytes, widely scattered cells, dark stained elastic fibers on light microscope
  • Has perichondrium and chondrogenic layer with few elastic fibers

Fibrocartilage

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• Collagen Type I, densely packed
  • Almost looks like dense regular CT
    • Diagnostic Characteristic: Has large round chondrocytes
• Not a lot of ground substance, replaced with type I collagen
• Chondrocytes secrete collagen and ground fibers
  • Don’t have to be epithelium to secrete
• Functions:
  • Pubic symphysis
  • Junction at attachment between tendon and bone

Intervertebral discs

• Fibrocartilage functions as a shock absorber
  • Annulus fibrosis – fibrocartilage
  • Nucleus pulposis – gelatinous matrix in the center
• Herniated disk
  • Fibrocartilage fails and nucleus pulposis is extruded, putting pressure on spinal cords and causing pain
  • Treated by keeping patient off his/her feet so that nucleus pulposis will suck back in
    • Wait for fibrocartilage to reform

Joints

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• Function in articulation
• Definition
  • Junction between two or more bones
  • Typically allows movement between bones

Functional Classificaiton

• Based on degree of movement
• Synarthroses – immovable
• Amphiarthroses – slightly movable
• Diathroses – freely moveable, most common

Structural Classification

• Based on type of tissue between bones
• Fibrous joints
  • Think layer of fibrous CT between bones
  • No cavity
    • Sutures – immovable bones in skull, synarthroses
    • Syndesmoses – articulating bones separated by dense fibrous tissue, amphiarthroses
    • Gomphoses – cone shaped socket; articulating bones separated by periodontal ligament, synarthroses
• Cartilagenous joints
  • United by cartilage
  • No cavity
    • Synchondroses – connected by hyaline cartilage, synarthroses
    • Symphyses – connecting material is a broad, flat disk of fibrocartilage, amphiarthroses
• Synovial joints
  • Ends of bone are covered by articular cartilage
  • Have a cavity
    • Gliding, hinge, pivot, ellipsoidal, saddle, ball-and-socket joints

Synovial Joints

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• Five common characteristics
  • Synovial cavity
  • Articular cartilage lines ends of bones
  • Have articular capsule
  • Synovial membrane
  • Synovial fluid is present within synovial membrane
    • Contains hyaluronic acid, lubricant
    • Nutrient supply to articular cartilage

Articular Cartilage

• Modified hyaline cartilage
• Functions to promote smooth movement between bones
  • Structurally designed to do so by lacking perichondrium on the surface
    • Naked cartilage on naked cartilage
    • Nutrients from synovial fluid produced by the synovial membrane that lines the cavity
      • Not really efficient at diffusing nutrients, but it does, slowly
  • Teflon function
• Made of type II collagen that does form arched fibers like canes
  • Help resists shear stress forces
• Additional support: cartilage near bone, away from articular cartilage is calcified
• Subchondral bone layer – increases strength

Repair of Joint Cartilage

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• Difficult to do:
  • Nutrient supply is poor
  • Articular cartilage doesn’t undergo mitosis
• To repair:
  • Allow condroprogenitor cells to enter fracture
    • Inject or drill holes to bone marrow
  • Move joint continuously to stimulate cartilage formation
    • Strap you to a machine that slowly moves you all the time
  • Repaired tissue is fibrocartilage
    • Not as smooth as before
  • Other forms of repair:
    • Subchondral bone grafts
    • Autologous bone grafts
    • Autologous tissue culture grafts
    • Injection of hyaluronic acid

Osteoarthritis

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• Degenerative arthritis affecting synovial joints
• Most common type of arthrisis
• Non-inflammatory and progressive deterioration of articular cartilage
• Characterized by:
  • Fibrillation of articular cartilage
    • Loss of ground substance and retention of fibers
  • Proliferation of cartilage at periphery
  • Replacement of cartilage spur with bone tissue to form bone spurs which reduce degree of movement

Rheumatoid Arthrisis

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• Inflammatory autoimmune disease
• Antibodies attack own joint cartilage and synovial membrane
• Primary inflammation of synovial membrane
  • Joints will be swollen
• Synovial membrane thickens and synovial fluid secretion increases
  • Joint swell up even more
• Pressure due to increased amounts of synovial fluid causes pain and tenderness
• Pannus of tissue forms across cartilage and erodes away cartilage
• Bone exposed and eventually fuses across joint